This invention relates to an improved method and apparatus for depositing polysilicon onto silicon substrates. More particularly, this invention relates to a single substrate processing chamber for depositing a polysilicon layer having improved uniformity onto semiconductor substrates and method therefor.
Doped or undoped silicon layers have been deposited onto silicon substrates, such as silicon wafers, using a low pressure chemical vapor deposition (CVD) process. A reaction gas mixture including a source of silicon, such as silane, disilane, silicon tetrachloride and the like, and optionally a dopant gas such as phosphine, arsine, diborane and the like, and optionally including a carrier gas such as hydrogen, is heated and passed over a silicon substrate to deposit a silicon film on the surface of the substrate. The exact crystallographic nature of the deposited silicon depends upon the temperature of deposition. At low reaction temperatures, the deposited silicon is mostly amorphous; when higher deposition temperatures are employed, a mixture of amorphous silicon and polysilicon or polysilicon alone will be deposited.
The prior art methods use comparatively low pressures of about 200-400 millitorr for this process. Good quality films can be formed, but very low deposition rates of about 100 angstroms/min for undoped, and about 30 angstroms/min for doped, polysilicon are obtained. This low deposition rate can be overcome by processing a plurality of wafers, i.e., up to 100, at once in a batch-type processing chamber.
However, present day thin film equipment for the semiconductor industry has been moving toward single substrate processing, because the processing chambers can be made smaller and processing can be better controlled. Further, modern semiconductor vacuum processing systems have been developed to carry out more than one processing step on a substrate without removing the substrate from a vacuum environment. The use of such vacuum systems results in a reduced number of particulates that contaminate the surface of the wafers during processing, thereby improving the device yield. Such vacuum systems include a central robotic transfer chamber connected to various processing chambers, such as the Applied Materials 5000 series processing system described in U.S. Pat. No. 4,951,601 to Maydan et al.
Thus CVD equipment for single substrate processing to deposit polysilicon onto semiconductor substrates are coming into commercial use. A CVD chamber for such purpose has been described heretofore and will be described with reference to FIG. 1.
Referring to FIG. 1, a single substrate reactor 31 has a top wall 32, side walls 33 and a bottom wall 34 that define the reactor 31 into which a single substrate 35, such as a silicon wafer, can be loaded. The substrate 35 is mounted on a pedestal or susceptor 36 that is rotated by a motor 37 to provide a time averaged environment for the substrate 35 that is cylindrically symmetric. A preheat ring 40 is supported in the chamber 30 and surrounds the wafer 35. The wafer 35 and the preheat ring 40 are heated by light from a plurality of high intensity lamps 38 and 39 mounted outside of the reactor 31. The top wall 32 and the bottom wall 34 of the chamber 30 are substantially transparent to light to enable the light from the external lamps 38 and 39 to enter the reactor 31 and heat the susceptor 36, the substrate 35 and the preheat ring 40. Quartz is a useful material for the top wall 32 and the bottom wall 34 because it is transparent to light of visible and IR frequencies; it is a relatively high strength material that can support a large pressure difference across these walls; and because it has a low rate of outgassing.
During deposition, the reactant gas stream flows from a gas input port 310, across the preheat ring 40 where the gases are heated, across the surface of the substrate 35 in the direction of the arrows 41 to deposit silicon films thereon, and into an exhaust port 311. The gas input port 310 is connected to a gas manifold (not shown) that provides one or a mixture of gases to enter the reactor 31 via a plurality of pipes into this slot. The locations of the input ends of these pipes, the gas concentrations and/or flow rate through each of these pipes are selected to produce reactant gas flows and concentration profiles that optimize processing uniformity. Although the rotation of the substrate and thermal gradients caused by the heat from the lamps 38 and 39 can significantly affect the flow profile of the gases in the reactor 31, the dominant shape of the flow profile is a laminar flow from the gas input port 310 and across the preheat ring 40 and the substrate 35 to the exhaust port 311.
In a typical process producing an undoped silicon layer on a silicon wafer, a pressure of about 80 Torr in a vacuum chamber is maintained by feeding hydrogen at about 10 liters/min into the chamber and adding about 500 sccm of silane at a temperature of the substrate of about 650xc2x0 C., as determined by a suitable pyrometer. A polysilicon film can be deposited under these conditions at a rate of about 2000 angstroms/min. The higher pressures used in the above method improves the rate of deposition of doped or undoped polysilicon.
While a great improvement in terms of deposition rate has been achieved using the above-described single substrate deposition chamber of FIG. 1 and a high pressure process, the uniformity of the deposited film and the variations in film thickness, sheet resistivity and the like from one substrate to another is not totally satisfactory. Thus it is desired to provide a single substrate CVD chamber to deposit polysilicon films having improved uniformity onto a substrate; and to improve the wafer to wafer uniformity of the substrates.
We have found that the uniformity of polysilicon films deposited on a substrate can be improved, and wafer-to-wafer variations can be reduced, by preventing processing gases from passing to the backside of the substrate support or susceptor. We found that deposits of solid materials onto the backside of the susceptor leads to non-uniformities of film thickness and film properties of the films deposited onto the substrates. It is believed these deposits cause temperature variations across the surface of the susceptor by varying the rate of heating of the susceptor, which lead to non-uniformities of film thickness, and, as the solids build up on the backside of the susceptor, temperature variations during sequential depositions occur that cause wafer-to-wafer variations in film thickness and film properties.
By providing a barrier to the flow of reactant gases to the backside of the susceptor, these backside deposits and temperature variations are prevented and more uniform polysilicon films are obtained. Wafer-to-wafer variations in film thickness and properties are also prevented or minimized.